of the Max - MDC

Mitochondrial Dynamics
Christiane Alexander
M
y group is studying disease genes that are essential for the integrity of mitochondria.
Mitochondria depend on their ability to form a dynamic network. Fusion and fission events of
the mitochondrial inner and outer membranes provide the molecular basics for such dynamics to
occur. Genes like OPA1 (optic atrophy 1) and MFN2 (mitofusin 2) are important for the fusion of the
mitochondrial membranes as well as the adaptation of the shape and the ultra-structure of mitochondria
membranes. Mutations in these genes lead to neurological disorders such as autosomal dominant
optic atrophy (adOA) and Charcot-Marie Tooth disease (CMT). We are studying the function of
these genes with the help of animal models, cell culture and biochemical approaches.
The expression of OPA1 is regulated via alternative
splicing, polyadenylation and proteolytic
processing
Vasudheva Rheddy Akepati, Maja Fiket
The OPA1 protein is a dynamin-related large GTPase, which
is associated with the inner mitochondrial membrane, facing
the inter-membrane space. By positional cloning, we
were able to identify the OPA1 gene, which spans about 100
kb of genomic sequence and is divided into 31 exons.
Alternative splicing of OPA1 leads to the generation of a set
of splice variants present in each cell of a given organism.
We discovered that during evolution, the number of alternative
splice variants increased from 1 transcript in yeast, 2
splice forms in Drosophila, 4 splice forms in mice to 8 forms
in humans. Moreover, the 3’UTR of OPA1 transcripts in
mammals contains multiple polyadenylation sites, which
may serve to confer differences in mRNA stability or translational
efficiency to the various isoforms. After translation,
proteolytic cleavage by different mitochondrial proteases
further adds to the very complex regulation of the OPA1
protein. Complete loss of OPA1 causes dramatic changes in
mitochondrial morphology, lack of respiration and a block
in fusion. Our work showed that isoform 1 is the most abundant
form at transcript and protein level. Therefore, we propose
that this form represents the ancient form of OPA1.
Loss of OPA1 leads to loss of cristae, respiration
and mitochondrial fusion in mice
Maja Fiket
In order to study the physiological function of OPA1 in mice,
we mutated the OPA1 gene by homologous recombination.
OPA1 +/- mice are phenotypically indistinguishable from wildtype
littermates, but OPA1 -/- embryos are reduced in size and
die during gastrulation. We observed that mitochondria of
OPA1 -/- embryos are completely fragmented. Similarly,
mouse embryonic fibroblasts derived from OPA1 -/- embryos
display fragmented mitochondria with abnormal inner membrane
morphology that are respiration and fusion deficient.
OPA1 -/- fibroblasts are less sensitive to staurosporineinduced
apoptosis and do not undergo cytochrome c
release. Transfection of OPA1 -/- fibroblasts with a cDNA
encoding the mouse homologue of human OPA1 isoform 1
restores respiratory chain activity, mitochondrial membrane
potential, tubular morphology and fusion of the mitochondrial
network. These reconstitution experiments assigned
thus distinct functions to one particular OPA1 isoform.
Manipulation of OPA1 affects fly neuroblasts and
glial cells but not neurons
Susanne Lorenz
Flies contain only two different isoforms of OPA1: a short
and a long form. Loss of OPA1 in the fly leads to similar
lethal effects during development as observed in mice.
OPA1 deficient flies do not pupate and stop growth at the
early third instar larval stage. Molecular transgenic techniques
allowed us to limit the expression of OPA1 transgenes
in the fly eye. The remaining tissues contained normal
OPA1 levels and garanteed for the rest of the fly’s body
to develop normally. We observed that the individual
expression of the short, long and mutated forms of OPA1 in
either the eye imaginal disc or just the photoreceptors of
the fly eye produced different phenotypical outcomes. A
devastating effect on proliferation in pre-mitotic tissue was
noticed after driving the expression of the short form or a
mutant form of OPA1. In contrast, post-mitotic photoreceptor
cells slowly degenerated after over-expression of the
long form or the mutated form. Further, we noticed a special
sensitivity of neuroblasts and glial cells to the over-
Function and Dysfunction of the Nervous System 175